Method of making light gage members of unalloyed low carbon steel sheets

ABSTRACT

Thermal hardening is applied to the material of light gage coldformed members made of unalloyed low carbon steel sheets having a carbon content not greater than 0.50 percent and a thickness not greater than 7 millimetres. Thermal hardening consists in quenching such steels from a temperature in the austenitizing range below the Ms temperature of starting martensite formation, holding the steel at such temperature until transformation is completed and then cooling it down to room temperature. By such method, an increase by 100 to 200 percent of the yield point may be obtained without distortion of the member. This seems to be due to the nose of the TTT-curve of beginning martensite formation being cut by the cooling curve so that softer crystal structures such as pearlite and bainite are formed which provide for absorbing residual stresses during martensite formation.

[451 Aug. 26, 1975 United States Patent 1 Mandoki OTHER PUBLlCATlONS METHOD OF MAKING LIGHT GAGE M M E S OF UNALLOYED LOW CARBON Metals Handbook, v01. 2 8th Edition, 1964, pages STEEL SHEETS 3646.

[75] inventor: Andor Mandoki, Budapest, Hungary Primary Examiner-R. Dean Attorney, Agent, or FirmYoung & Thompson [73] Assignee: Licencia" Talalmanyokat Ertekesito Vallalat, Budapest, Hungary ABSTRACT Thermal hardening is applied to the material of light gage coldformed members made of unalloyed low carbon steel sheets having a carbon con tent not 2 7 9 l 8 a1 0 03 o N DI F p FA o tH 22 Related U.S. Application Data greater than 0.50 percent and a thickness not greater Continuation of Ser, No 815,889, A

m My 1969, than 7 millimetres. Thermal hardening consists in quenching such steels from a temperature in the austenitizing range below the Ms temperature of starting abandoned.

[30] Foreign Application Priority Data martensite formation, holding the steel at such tem- Apr. 19, l968 MA 1829 perature until transformation is completed and then cooling it down to room temperature By such Ce y hhTb teln f u 0 C nh mm m w o r0 10 Ms Pi G l O t ome 2 hm h mwmm mm me m mme bm m l w o S m a m m cymn msm i n e a Tb lm f d e o b mmmv w m mc @912, 4M5 A 42 1 6 3 46 W M 4 ,3 l 35 m 2 7 4M 8 n 4 U .8 H m4 mwh c Hr "H ms 0 s & .M .m UhF H M 555 [ll ures such provide for absorbing residual stresses during martensite forma tion.

the cooling curve so that softer crystal struct as pearlite and bainite are formed which [56] References Cited UNITED STATES PATENTS 3,4lO 734 ll/I968 Taylor.. ....lm....l.,. l. l48/l43 10 Claims, 8 Drawing Figures ll ll lhl l w l i L,

l lllllll llll'll l. [Jil time SHEET 1 of 2 time PATENTED 2 5 I975 METHOD OF MAKING LIGHT GAGE MEMBERS OF UNALLOYED LOW CARBON STEEL SHEETS This is a continuation of application Scr. No.

85,889, filed Apr. ]4. i969, and now abandoned.

BACKGROUND OF THE INVENTION This invention refers to methods of making light gage members of unalloyed low carbon steel sheets by cmploying thermal hardening.

Thermal hardening is generally substituted for conventional quenching and tempering in order to obtain improved mechanical properties such as high ductility or notch toughness at a given high hardness and/or to decrease the likelihood of ckracking and distortion and thus to employ only two processing steps instead of conventional quenching and tempering which is a three step operation.

Various methods of thermal hardening of steel have been developed and are known in the art such as austempering. martempering or marquenching and patenting.

Austempering of steel means to heat it to a temperature within the austenitizing range from where it is quenched in a bath of constant temperature above the Mr point or temperature of rapid conversion of austenite in martensite. The steel is maintained in the bath at such temperature until the austenite is isothermally transformed to bainite whereafter it is left to cool down to room temperature. usually in still air. Austempcring is described in detail in the specification of U.S. Pat. No. 1324,09) to Edgar C. Bain et al. which is hereby incorporated by reference. Austempering has modified forms as well in which bainite is mixed with pearlite. Another form of modified austempering is patenting which is the heat treating of wires or rods providing a combination of moderately high strenght and high ductility.

As to martempering. it consists in quenching a steel from the austcnitizing range to a temperature in or slightly above the martensite range and holding the steel at such temperature until its whole cross sectional area is uniformly heated whereafter the steel is permitted to cool at a moderate rate. Hereby. fairly uniform martensite formation is obtained throughout the work piece and yet excessive amounts of residual stresses are avoided. Martempering has likewise its modified forms which are distinguished from standard martempering only by a lower temperature of the quenching bath. Such modified martempering permits to employ faster cooling rates which is important for steels of lower hardenability.

All these methods of thermal hardening are described in detail in Metals handbook" of the Ameri can Society for Metals. Metals Park. Ohio. 8th edition. l964. vol. 2. hereby incorporated by reference which has widely been taken recourse to when composing the present text.

As is known. with austcmpering a work piece is quenched past the nose of the TTT-curve (timc temperature transformation curve). If the cooling curve intersects the nose of the TIT-curve true austempering is already impossible. Such intersection is unavoidable ifthe carbon content of the steel is too low and its surface-to-mass relation too little since then extremely fast pearlite reaction times have to be reckoned with and the cooling curves cannot avoid the nose of the TTT-curve of starting conversion either practifit I cally (too small time allowance for quenching) or theoretically (no such allowance at all). Due to such limitations. the domain of plain carbon steels adaptable to austempering is restricted to carbon contents not less than 0.50 /1 carbon up to 1.00 71 and a minimum of 0.60 mi manganese. By increasing the austenitizing tcmpcrature the nose of the Tf'l curve could be caused to shift to the right whereby compositions or section sizes that would otherwise be border line for austempering would be enabled to be processed. However, coarser grain si7cs resulting from increased austenitizing temperatures might entail deterioration of otherwise desired mechanical properties such as ductility and yield point. For similar reasons. carbon steels of lower carbon content will be restricted to proportionately lesser thicknesses and 5 millimetres are about the upper limit.

With modified practice of austempcring, the process differs from true austempering in that the quenching rate is not rapid enough to avoid the nose of the TTT- curve which is intersected by the cooling curve substantially above the Ms temperature so that fine pearlite will be formed.

Likewise. steels chosen for martempering must contain sufficient amount of carbon to move the nose of the TTT-curve to the right so as to allow sufficient time for quenching a work piece past it. Actually steels selected for martempering must have a carbon content somewhat higher than for conventional (uninterrupted) quenching which means again that plain car bon steels of lower carbon content are unsuitable for both standard and modified martempering since with such steels the nose of the TTT-curve would be intersected and the formation of martensite would be frustrated. For similar reasons. the thickness of the work piece must not exceed an upper limit (generally 5 millimetres) either.

Thus. according to the prios art. thin steel members of low carbon content which cannot be hardened by the conventional method of austenitizing, quenching and tempering because of their low carbon content and the danger of distortions caused by residual stresses, cannot be thermally hardened by standard or modified austempering either the use of which is restricted to plain carbon steels having a carbon content more than 0.50 percent. Moreover they cannot be thermally hardened by martempering. standard or modified. either because of their low hardenability.

On the other hand. it is known that unalloyed low carbon steels are considerably less expensive than simi lar steels of higher carbon content. Therefore. it would be desirable to process such steels into light gage coldformed members and particularly in sections since there is great demand for disconnectable metal struc turcs composed of such members and erectable in situ. Obviously. considerable savings as to costs. weight and space might be obtained ifsuch metal structures could be made of light gage cold-formed members of low car bon content. Hitherto. however. unalloycd low carbon stccls have been excluded from the use for such purpose because of their relatively low yield point and more particularly due to the poor relation between yield point and associated tensile strength which are the basis of dimensioning such structures.

SUMMARY OF THF. INVENTION The main object of the present invention is to provide a method by which the yield point of such members can considerably be improved so as to permit their use in constructions which hitherto have required steels of higher carbon contents. The invention is based on the discovery that contrary to the view mirrored in the prior art. even such steels can be thermally hardened without detrimental consequences if. on the one hand. the nose of the TTT-curve of beginning transformation is not only intersected but deliberately crossed by the cooling curve and. on the other hand. quenching is pushed beyond the Ms temperature yet not far enough to reach the temperature range of conventional quenching responsible for the formation of brittle martensite. Sufficient time is then allowed for the conversion of austenite still present to be converted in martensite forming at such temperature whcreafter the member is permitted to cool down to room tempera ture.

At present. it is believed that the favourable results as to increased yield point and absence of distortions obtained by such method are due to the formation of substantial amounts of less brittle martensite beneath the Ms point and to the formation of softer crystal structures such as pearlite and bainite during the cross ing of thc TTT-curve nose prior to the formation of martensite. softer crystal structures and martensite being in favourable mutual proportions probably due to the cooling curve being steeper because of lower quenching temperature (as if the first section of the cooling curve had been rotated in the clockwise direction). Thus. the method according to the invention is intermediate between austempering and modified martempering in that final temperature of quenching lies beneath the Ms point as distinguished from austempen ing. and in the course of quenching softer crystal structures are formed because of crossing the nose of the TTT-curve of beginning transformation as distinguished from modified martempering (see e.g. FIG. I Austempcring' on page 57 and FIG. 1 Modified martempering on page 37. respectively. of "Metals Handbook"). Experiments have proved that such crossing of the nose of the TTT-curve of beginning transformation is ensured if members subjected to the suggested thermal hardening are made of unalloyed carbon steel sheets the carbon content of which is not greater than the aforesaid critical value of 0.50 percent and the sheet thickness is not greater than 7 millimetres which means that even the thickness range could be extended beyond the hitherto known upper limit of about 5 millimetres.

summarily. the invention is concerned with a method of making light gage cold-formed members and more particularly sections of unalloyed low carbon steel sheets having a carbon content not greater than 0.50 percent and a thickness not greater than 7 millimetres wherein the improvement consists in the step of thermally hardening the members by means of quenching the same from a temperature above the critical temperature range to an intermediate temperature below that at which rapid conversion of austenite to martensite is obtained but above that at which conversion is not rapid enough to cause the austenite to be converted in brittle martensite. and thereafter maintaining the quenched steel at this intermediate temperature for a time interval sufficient to obtain conversion of the re maining austenite to pure martensite characteristic for the temperature of heat treatment. and then cooling to atmospheric temperatures. The term cold-formed (itl member" in the present specification and in the appended claims is meant to cover individual constructional parts as well as small. thin sections made by bending sheet or strip steel in roll-forming machines. press brakes or bending brakes. Furthermore. the term sheet is meant to cover plates and strips as well. the difference being. principally. a matter of thickness and width of material whether or not the material is furnished in flat form or in coils.

BRIEF DESCRlPTlON OF THE DRAWING The invention will be described hereinafter in closer details. by taking reference to the accompanying draw' ings in which:

FIG. 1 shows a time-tempcrature transformation diagram symbolic ofthe thermal hardening employed with methods according to the invention.

FIG. 2 is a similar diagram showing actual TTT- curves of various steels having carbon contents within the range prescribed by the invention.

FIG. 3 shows a member made by a method according to the invention FIGS. 4 to 8 show sections of various shapes.

Same reference characters refer to similar details throughout the drawings In the drawings. FIG. 1 illustrates the basic principle of the method according to the invention. Lines l and ll represent broadly the envelopes of TfT-curves of starting and complete transformations. respectively. critical temperature. A03 represents the range of critical temperatures of steels having carbon contents within the scope of the present invention. Line Ms is a respective symbol of temperatures below which martensite formation is due. 0 refers to the cooling curve characteristic ofqucnehing such steels from above critical temperature A03 below Ms temperature. then maintaining it at constant temperature and. finally. cooling it to room temperature. It is seen that the envelope l of'l'fT-curves of beginning transformation is already cut at its nose. Consequently. it is practically im possible to quench such steels rapidly enough to pass the nose of their 'lT'l curvcs of beginning transformation. Thus. formation of other crystal structures than martensite will take place prior to martensite formation. On the other hand. an intersection with the envelope ll of TTT-curves of complete transformation has to be avoided. otherwise only little martensite formation would occur. Experiments have shown that such requirement will be met with if sheet thicknesses greater than 7 millimetres are dismissed. Thus. with steel members having a carbon content not greater than 0.50 percent and a thickness not greater than 7 millimetres. the cooling corve Q will always cross the nose ofthe respective TTT-curve of beginning transformation without intersection with the associated TTT- curve of complete transformation as illustrated for the envelope I both branches of which are intersected by the cooling curve Q. the latter being steep enough not to intersect envelope ll.

The significance of such double intersection or crossing is that at the first intersection A the austenite will begin to convert in softer crystal structures in correspondence with decreasing temperature until the second intersection l! is reached. In this period. pearlite and bainite will be formed. After the second intersection 8 the remaining austenite will stay unchanged until the Ms temperature is passed (point (i whereupon conversion of the remaining austenite into martensite will set in. Since such conversion takes place at a con stant temperature broadly between 150 and 470 centigrade elevated with respect to the temperature where brittle martensite is formed (as with conventional quenching). the remaining austenite becomes converted into relatively soft martensite. conversion being completed at the intersection of the horizontal section of the cooling curve with the envelope II of TTT-curves of complete transformation (point D]. Thereafter. cooling to room temperature may be carried out along the final section of the cooling curve 0. a substantial allowance of time to start. such cooling being provided for with regard to the uncertainty of the course of the TTT-curves of complete transformation at such medium temperatures. Thus. the crystal structure of the member will consist, besides of softer elements such as pearlite and bainite formed between points A and B. of a substantial portion of soft martensite. The former are believed to account for the absence of distortions during the formation of the latter whereas the presence of substantial amounts of soft martensite seems to be responsible for an unsuspected increase of the yield point. Obviously. the presence of bainite contributes to the increase of the yield point and. vice versa. the formation of soft rather than brittle martcnsite may result in avoiding residual stresses. Thus. increased yield point will be obtained without significant distortions which is the main object of the present iiivcntion.

FIG. 2 shovvs the actual 'ITT-curves of beginning and complete transformations for steels having carbon Con tents of (L50 9? (dash-and-dot lines). (1.35 72 (dotted lines) and (H if (dashed lines). respectively. and. thus. covering the whole range of pertinent steel compositions. It is seen that the noses of all T'I'I -curves of beginning transformation are crossed by the cooling curve 0. and none of the TTT-curves of complete transformation is intersected by the same which are the theoretical conditions for the working of the method in accordance with the invention DESCRIPTION OF THE PREFERRED EMBODIMENTS Various examples of steels selected and heat treated or thermally hardened in compliance with the present invention are set forth in the following four tables which group altogether nineteen embodiments stating decisive data of heat treatment and characteristic data of mechanical properties. All steels are plain carbon steels with the usual amounts of silicon. sulphur and phosphorus. They differ from one another as to their contents of carbon and manganese. which rule the subdivision into groups. The lowest carbon content is (l. l 5 percent for steels of group l. The two next groups include steels of carbon contents of (I25 and 0.35 percent. respectively. Finally. group IV lists steels having a carbon content of (I45 percent which is almost the maximum amount of carbon permissible with steels selected for the purpose of the invention. The content of manganese correspondingly decreases as the carbon content approaches its upper limit though. obviously. other manganese contents might be chosen within the limits of such constituent in plain carbon steels as will be apparent to the skilled art worker.

GROUP l OF EXAMPLES Carbon: (l. l 5 9i Manganese: (L80 '7 Example I 2 3 Thickness in mm (L5 1.5 2.5 Austcnitizing temperature in 951) 95() 950 Ms temperature in C Jot) 460 460 Temperature of quenching in 40(1 35(1 350 Theoretical quenching time in sec 30 55 50 Actual quenching time in sec ZUI) 3H0 300 Initial tensile strength in leg/mm 37 37 37 Final tensile strength in kg/mm" 8U 7U fill Initial ductility in "/1 30 3U 30 Final ductility in '/1 I8 "2 Initial yield point in lag/mm 22 22 22 Final yield point in kg/mm" (ill 5U 4U Perccntual increase of yield point in "I; I7 I27 82 GROUP ll OF EXAMPLES Carbon: (I Manganese (L65 /1 Example 4 5 o 7 K Thickness in mm .1) .(i .(l 2,1) .(I Tl) Austenitiying temperature lit f JIU )lll lll )IU JIII 9H) Ms temperature in "C 421i 420 420 420 42H 42() Temperature of quenching III I7Il 2H0 25H llll 35H Jill) Theoretical quenching time in sec 30H 25() IXII ll(l b5 Actual quenching time in sec (iUII (WUI) 5lltl 4(IU .IIII) KIIII Initial tensile strength in ltg/nim 45 45 45 45 45 Final tensile strength in kg/mm I25 I I5 H15 9U 8U 7U Initial ductility in 'r' 2U 2U 2U 2U 2U 2(I I-inal ductility in '4 5 o 7 3 14 lo Initial \ield point in kgimm 25 25 25 25 25 25 Final yield point in Its/"Int )(J 85 Ht) 7() (\(l Pcrcentiial increase of yield point in V? 260 2-III 221) It) l-Itl I20 (iR()L'P III OI EXAMPLES Carbon: (L35 I Manganese: (L a Etample It) I I l2 l3 14 I5 Thickness in mm 2.1) 2.0 1.0 III (l 6.0 Austcniti/ing temperature In C K H'III 87H XTII I471) HTII Ms temperature in 3X5 3X5 3K5 385 385 Temperature of quenching in t Run 35a Run -iuu 25o llicorctical quenching time in sec lZtl 75 It! IZII JII It! Actual quenching time Ill sci: JHII Fill) 5H0 -Ill(l 3U 5H0 Initial tensile strength in kg/mm 5 55 55 55 55 5 Final tensile sttcngtii iii kgxmm ll5 lli5 III5 9 HI) I Initial ductility in W I7 I? l7 l7 l7 l7 Iinal ductility in r k I HI l2 l4 lo Initial \icltl point in lsg/nini (I 3t) 3t) 30 3t] 3|) -Continucd GROUP III ()I EXAMPLES Carbon: (1.35 "I Mangancsc: ,i

E\amplc III II II I3 l-I I5 Final yield point in leg/mm KS X0 8U 7U btl on Pcrccntual increase ofyicld point in i I84 I67 lh7 I34 llill Hit) (iROlP IV OF EXAMPLES ('arhon: 0.45 OI Manganese: USU? lma\i|num) Fxamplc lo l7 IS I) Thickness in mm 2.0 I. 41) 7.II Austcnitiying temperature in f P45ll 85H N50 XSU Ms temperature in 350 350 k) 350 l'cnipcraturc of quenching in 33H 331) 330 :50 Theoretical quenching time in .scc I31) I31) I I kill Actual quenching time in set. 4H0 If) 4H0 SlIlI Initial tensile strength in kg/mmF b5 n5 ns (15 Final tensile strength in kg/mm" HS 115 IIJS 91) Initial ductility in '1 l4 l-1 14 I4 Final ductility in 'i 9 H l1 l4 Initial yield point in hg/ltllfl 35 5 35 Final yield point in kg/nim )l) 95 til) 7!) lcrccntual increase of \icltl point in vi I57 I l) l2) ZIIU In each ofthe nineteen cases. conventional salt baths have been used for quenching. Such baths are descriged e.g. in the above mentioned Metals Handbook.

Furthermore. in each case final cooling has been carried out in still air though water might be used as well. Then. a further increase of the yield point by 2 to 3 kilograms per square millimetres can be obtained due to the traces of remaining austcnite being converted in brittle martensitc.

The tables show that each embodiment is characterised by a considerable increase of the yield point with respect to values prior to heat treatment. Though such increase is obtained through the whole range of pertinent steel compositions, greatest values are associated with thicknesses between 1.5 to 4 millimetres (see group II. examples 4. 5 and 6 and group IV. examples l6 and 17) and/or with a carbon content not less than U.l5 percent (see group II. examples 4. 5 and 6).

Furthermore. the tables show that manganese con tents not less than 0.5 percent have proved very favourable as to the percentual increase of the yield point as witnessed by all groups of examples.

The temperature of heating will preferably amount to 30 to 90 ccntigrade higher than the lowermost temperature limit of the austenitizing range. Though, as has been referred to. temperatures higher than that increase thc quenching speed and thereby the strength. their grain coarsening effect outbalances the gain obtained by higher quenching speed. One of the favourable features of the invention is that usual austcnitizing temperatures may be employed for obtaining the dc sired results.

FIGS. 3 to 8 show various members made by the method according to the invention. Basically. two

(ill

groups of such members may be distinguished. One group comprises individual parts such as the roller chain link plate shown in FIG. 3. The other group includes various sections. such as flat sections (FIG. 4). equal lcg angles (FIG. 5). open or closed channel sections (FIGS. 6 and 7, respectively) or sections of special cross-sectional shapes such as shown in FIG. 8.

Flat sections as illustrated in FIG. 4 may be heat treated according to the present invention in continuous operation. Then. the material is furnished in coils and is passed through various treating stations in band form. The heat treated band may be wound up for storage or cold-formed and cut to predetermined lengths. In both cases heating. quenching and cooling is applied to the work material prior to shaping the crosssectional area thereof.

On the other hand. individual parts such as the roller chain link plate shown in FIG. 3 or the sections shown in FIGS. 5 and 8 will preferably be heat treated groupwise. In such cases. the work material will be shaped or cold-formed and subdivided into predetermined lengths by means of cutting (see FIG. 6). prior to heating. quenching and cooling.

With both methods. the equipment necessary for continuous or groupwise heat treatment may be composed of units well known in the art for similar purposes and in a manner familiar with the skilled art worker. In this respect. reference is again taken to the above mentioned Metals Handbook.

I claim:

1. In a method of making light gage members of unal- Ioyed low carbon steel sheets having a carbon content not greater than (1.50%.

the steps of heating a said member above the critical temperature thereby to form austenitc.

thermally hardening the member by quenching the same from a temperature above the critical temperature to an intermediate temperature of substantially ISO" to 470 centigrade along a cooling line that crosses the nose of the time-temperature transformation curve. to convert only a portion of said austenite to pearlite and bainite.

and thereafter maintaining the quenched steel at this intermediate temperature for a time interval sufficient to obtain conversion of the remaining austenite to pure martensite characteristic of the temperature of heat treatment.

and then cooling to room temperature.

2. In a method of making members as claimed in claim I including the step of cold-forming the crosssectional area of work material the further improvement of applying the heating. quenching and cooling to the work material prior to cold-forming ol' the crosssectional area thereof in a continuous operation.

3. In a method of making members as claimed in claim I including the step of cold-forming the crossscctional area of work material the further improvement ofapplying the heating. quenching and cooling to the work material groupwisc after cold-forming its cross-sectional area.

4. In a method of making members as claimed in claim I the further improvement of employing steel sheets of a thickness of IS to 4 millimetres for work material.

5. In a method of making members as claimed in claim 1 the further improvement of employing steel 8. ln a method of making members as claimed in claim I the further improvement of effecting the final cooling in water.

9. In a method of making members as Claimed in claim I. the further improvement of performing said quenching in a salt bath.

]0. In a method of making members as claimed in claim I. the further improvement of maintaining the thickness of the members not greater than 7 millimeters. 

1. IN A METHOD OF MAKING LIGHT GAGE MEMBERS OF UNALLOYED LOW CARBON STEEL SHEETS HAVING A CARBON CONTENT NOT GREATER THAN 0.05%, THE STEPS OF HEATING A SAID MEMBER ABOVE THE CRITICAL TEMPERATURE THEREBY TO FORM AUSTENITE, THERMALLY HARDENING THE MEMBER BY QUENCHING THE SAME FROM A TEMPERATURE ABOVE THE CRITICAL TEMPERATURE TO AN INTERMEDIATE TEMPERATURE OF SUBSTANTIALLY 150* TO 470* CENTIGRADE ALONG A COOLING LINE THAT CROSSES THE NOSE OF THE TIME-TEMPERATURE TRANSFORMATION CURVE, TO CONVERT ONLY A PORTION OF SAID AUSTENITE TO PEARLITE AND BAINITE,
 2. In a method of making members as claimed in claim 1 including the step of cold-forming the cross-sectional area of work material the further improvement of applying the heating, quenching and cooling to the work material prior to cold-forming of the cross-sectional area thereof in a continuous operation.
 3. In a method of making members as claimed in claim 1 including the step of cold-forming the cross-sectional area of work material the further improvement of applying the heating, quenching and cooling to the work material groupwise after cold-forming its cross-sectional area.
 4. In a method of making members as claimed in claim 1 the further improvement of employing steel sheets of a thickness of 1.5 to 4 millimetres for work material.
 5. In a method of making members as claimed in claim 1 the further improvement of employing steel sheets having a carbon content not less than 0.15 percent for work material.
 6. In a method of making members as claimed in claim 1 the further improvement of employing steels for work material the manganese content of which is 0.5 to 1.0% by weight.
 7. In a method of making members as claimed in claim 1 the further improvement of carrying out quenching from a temperature by 30* to 90* centigrade higher than the lowermost temperature limit of the austenitizing range.
 8. In a method of making members as claimed in claim 1 the further improvement of effecting the final cooling in water.
 9. In a method of making members as claimed in claim 1, the further improvement of performing said quenching in a salt bath.
 10. In a method of making members as claimed in claim 1, the further improvement of maintaining the thickness of the members not greater than 7 millimeters. 